جستجوی مقالات مرتبط با کلیدواژه « sediment transport » در نشریات گروه « عمران »

Coarse particle motion behavior plays a crucial role in sediment and hydraulic engineering, though its physics is still not fully understood. Disregarding the inherently stochastic nature of the sediment transport leads to various equations for bedload transport which are now being challenged due to their poor results. By applying sensors, like accelerometers and gyroscopes, particle transport physics could be carried out in a more scrutinized approach. In this study, an instrumented synthetic particle ("so-called" the smart particle) was designed and applied (with different densities) in sets of laboratory experiments that covered a hydraulics domain between low transport regime (near- and above-threshold) and higher transport regime (above threshold with a relatively high Reynolds number) conditions. Using the instrumented particle (smart-particle) could bring opportunities to learn more about the physics of the bed particle transport in rivers for different regimes and could bring data in hand for instantaneous particle changes throughout time (hear 0.004 seconds used for data sampling). Therefore, the kinetic energy as a parameter that delivers the behavior of particle energy interaction between the exposed particle and its surroundings (flow and the bed particles) was chosen to be studied. Since the dynamic features of the particle in transport are stochastic, the probability distribution functions, which could describe the particle behavior, were selected (Weibull, Lognormal, Normal, and Gamma distributions). In this case, it was shown that the Weibull distribution best described the particle kinetic energy in lower transport regimes, while for a higher transport regime, the Log-normal distribution worked better. Furthermore, the energy signals of the particle moving throughout the flume for different transport regimes were derived, and it was shown that the average energy gain and loss of the particle decreased exponentially as the particle Reynolds number increased. The presented results here could also be applied in similar hydraulic conditions in eco-hydraulic topics, specifically macro-plastic movement as bedload in river courses and the Aeolian research.

Laboratory experiments were carried out to investigate the hydraulics, turbulence structure, and sediment removal capacity of bridge pier with different adjusted roughness. Four experimental models of the roughness were employed to measure the effect of the geometry of the roughness on the scour hole formation and depth. The effects of roughness geometry on the turbulence structure and local erosion through the bridges were studied for one sand sediment size. The three-dimensional velocity profiles and turbulence characteristics of flow downstream and upstream of the bridge pier were measured by Acoustic Doppler velocimetry (ADV) technique due to different adjusted roughnesses. After filtration process by WinADV software, the contour plots of the turbulence kinetic energy were depicted due to triangular roughness. The maximum longitudinal eroded section, which is commonly located by bridge pier corners, was measured through the experiments. The results of the scour hole measurements indicate that due to increasing the length of the roughness, the scour hole depth values decreased; however, increasing the vertical distance has the same effect on the scour hole formation. Furthermore, different geometries of the employed roughness illustrate that the triangular roughness has less scour hole depth than other experimental models with the same hydraulic condition. The detailed information of the magnitude, distribution, and probability of turbulence structures was extracted from time-series data using power spectra. The turbulence data were compared with the topography of upstream erosion. Analysis of the power spectrum density function indicated that the discontinuous adjusted roughness remarkably reduced the downward energy level of vortexes at the upstream of bridge pier, which is supposed that the gap opening through the roughness can increase downward jets impact. It appears that this condition reduces the turbulence kinetic energy near the bed elevation at the upstream face of squire bridge pier.

Sediment motion behavior plays an important role in the sediment and hydraulic engineering, though its physics is still not fully understood. Ignoring the stochastic nature of the sediment transport leads to various equations for bedload transport which are now being challenged due to their results. In this study, the non-suspended particle motion (bedload transport) in different hydraulic conditions was assessed by a particle tracking technique called the Particle Tracking Velocimetry (PTV). The results of the PTV were applied to describe the particle behavior throughout the probability distribution functions. Knowing the particle motion behavior would be a guidance to learn more about the parameter/s governing the particle transport in different sediment transport regimes. After calibrating and validating the frames (resulted from the PTV), the instantaneous particle velocity was measured. Different probability distribution functions were assessed with Kolmogorov-Smirnov criterion to find the best function which fits the collected data (i.e. the particle velocity). It was shown that the probability distribution function is Log-Normal for lower particle Reynolds number and on the other hand, in the higher particle Reynolds number, the Normal distribution is best describing the particle velocity. The results of this research also could be applied in similar hydraulic conditions in eco-hydraulic field, specifically macro-plastic movement as bedload in river courses.

One of the common phenomena in water and urban wastewater transport pipes is sedimentation. Due to complexity of sedimentation phenomenon and influence of various parameters, determining the governing equations are difficult and classical mathematical models are not sufficiently accurate. In this study, capability of Gaussian Process Regression (GPR) in predicting sediment discharge in circular rainwater transport pipes with smooth and rough beds was investigated. In this regard, hydraulic and sediment parameters which had most correlation with sediment discharge were determined using factorial analysis. Then, different models were developed and using three experimental data series were investigated. Then, the accuracy of the results was compared with the classical sediment transport models. The results showed the high efficiency of the intelligent GPR model in prediction of sediment discharge in rainwater transport pipes compared to the classical methods. In both pipes the model with input parameters λs, Fm, Dgr, d50/y which are relative sediment size, non-dimensional sediment size, fraud number of sediments, and total roughness coefficient, respectively, was obtained as superior model. It was also observed that pipe diameter affected the models efficiency and with increasing pipe diameter, model accuracy increased. The pipe with a diameter of 305 mm led to more accurate results. Factorial and omitted sensitivity analysis showed that d50/y was the most effective parameter in estimation of sediment discharge in both smooth and rough pipes.

Sand and gravel are essential materials for developing purposes in infrastructures and many various purposes. Iran, is a developing country and numerous infrastructure projects all around the country are under construction. This issue, demonstrates the growing demand for sand and gravel harvesting. Irregular and non-technical harvesting of sand and gravel from rivers, plays an important role in unwanted morphological and environmental side-effects. Physical modeling and numerical simulation are two main techniques to investigate this phenomenon. Considering the high cost of constructing physical models, application of numerical tools for simulation of hydrodynamics and sedimentation has made a significant help for understanding the related phenomenon including the effects of sand and gravel removal in different rivers. In this study, the accuracy of the MIKE21 as a two-dimensional numerical tool, in simulation of sand harvesting hole displacement was investigated by comparison with laboratory data. For this purpose, nine experiments with different dimensions of excavation holes were designed in a 10 m long and 0.7 m wide laboratory flume with uniform sand bed materials. (D50=0.71mm). Two types of triangular and trapezoidal excavation holes were tested. Four important point plus depth and area of the excavated hole were considered as base points of comparison between simulated and experimental results. The flow depth was constant during all experiments (12 cm) and clear water condition was considered (v/vc=0.95). Acceptable agreement between numerical and experimental results was observed. However, the accuracy of the model was more in larger holes whereas the maximum error in predicting the migrated hole geometry in trapezoidal holes was about a half of triangular ones. After verifying the numerical model in laboratory, a specific reach in Helleh river was considered as a case study. Initially one-dimensional model of the river was simulated with HEC-RAS. 25 years return period flow hydrograph was introduced as the upstream boundary condition. Normal flow depth at Helleh Lagoon and time series of the water surface elevation changes of the Persian Gulf were introduced as two downstream boundary conditions. The boundary conditions of the selected reach for   two-dimensional modeling were extracted from one-dimensional simulation. After setting up the two-dimensional model, the effect of sand and gravel mining on a bridge in the reach was investigated in two different scenarios. The distance of the sand mining hole to the bridge was selected as 1000 m and 100 m respectively in two scenarios. It should be noted the simulation was conducted only for a 25 years return period within 16 days. More severe floods can leave more significant effects on the river and in-line structures. The results indicated that for a flood event with a return period of 25 years which was considered for simulation, sand and gravel mining had changed the hydraulic parameters and bed profile significantly, so that the flow depth at the vicinity of the excavation hole was raised up to 77% in second scenario. The flow velocity was reduced up to 75% in the first scenario and the bed profile was decreased up to 1.27m at the foundation of the bridge in the second scenario. Initial signs of river meandering were emerged in the second scenario where the flow was deviated to the mining hole.

In rivers with a compound section, typically the relatively high roughness of floodplains, as compared with the main channel, grounds the speed difference in these two ( main channel and floodplains). The velocity dissimilarity also creates shear layers at the interface point of the main channel and the floodplain. The development of shear layers will also cause turbulence in the mutual plane of the main channel and the floodplain. In such conditions, the average flow speed cannot be applied to compute parameters such as shear stress, bed-load discharge, and so forth. Laboratory experiments was carried out to investigate the effects of floodplain divergence on flow hydraulic and the rate of sediment transport through roughness and different divergent angles. This research 36 tests were done at a concrete flume with compound channel cross section (9 and 27 tests with prismatic and Non-prismatic cross section respectively). By using a micro-propeller and ADV (with 200 Hz frequency) in different sections the velocity were measured. For depth ratio of 0.15 0.25 was used the micro-propeller and depth ratio 0.35 was used the ADV. The results reveal that the increase in all three factors of roughness, divergence angle, and relative depth cause dramatic changes in hydraulic of flow and sediment transport.

Sediment transport by a fluid flow is one of the most important two phase flows in the nature.
Predicting river behavior at bends is important since rivers rarely run on straight paths in nature, and most rivers have meandering forms. In a river bend the presence of centrifugal force leads to the formation of secondary flow. As a result water particles near the surface are driven outward and the flow at the bed of a channel is directed toward the inner bank. Therefore locating the intake at outer bank of bend is one of the ways to reduce sediment intern to the lateral intake.
Due to the existence of secondary current in channel bends, the mechanism of flow and sediment transport is much complex whereas locating lateral intake at outer bank of the bends decreases this complexity.
Lateral intakes are hydraulic structures, used for water conveyance for domestic, agricultural and industrial purposes, characterized by a very complex three-dimensional morphodynamic behavior: since streamlines near the lateral intake are deflected, some vortices are formed; also, the pressure gradient, shear and centrifugal forces at the intake generate the flow separation and a secondary movement responsible for local scour and sediment deposition.
Knowing the mechanisms of sediment transport in the channel bends with lateral intake and the manner in which sediments can enter the diversion can help engineers in the better design of these hydraulic structures.
In this paper, sediment transport phenomena and mechanisms of sediment entry to the lateral intake were carried out in a 180 degree channel bend using injection of sediment on rigid bed. In this order, a sediment injection device with uniform and adjustable rate were designed and made. The intake were installed in position 115 degree of bend with 45 degree diversion angle according to previous research suggestion. The sediment injection device were fixed at upstream of the bend and dried sediment with the rate of equal to the capacity of the channel upstream of the bend were injected.
In order to analyzing the results, three components of flow velocity were measured using 3D Acoustic Doppler Velocimeter (Vectrino). Moreover, the sediment movement path in different times and diverted sediment ratio to intake are measured.
In this research, effect of Froude Number and diversion discharge ratio on sediment transport in the bend and mechanism of sediment entry to the intake were considered. The results showed that in different Froud number and diversion discharge rate, after equilibrium time, a sediment accumulation occurred in section 45 degree of bend and another sediment accumulation occurred in section 135 degree of bend near inner bend. The reduction of Froud number caused development of bed forms and their heights. Also the results showed the mechanism of sediment entry to the lateral intake is affected by diversion discharge ratio. In Qr=40% (and Fr=0.38), the mechanism of sediment entry were consisting of continues entrance from downstream edge of intake and periodic entrance from upstream of the intake but in Qr=25% (and Fr=0.38), the mechanism of sediment entry were only consisting of continues entrance from downstream edge of intake.

Use of non-core rock-fill dams is one of the structural methods for flood control. Flood flows are usually known to have high sediment loads, and carry high content of sediment into the body of rock-fill dam with large pores that characterize the body of these types of dams. It is thus essential that the sediment concentration be determined at different points of the body so that one could determine the critical points of sedimentation, the amount of sediment passing and the amount of sediment trapped. Therefore, in the present study, a mathematical model of flow simulator was implemented in MATLAB, on the basis of Saint-Venant equations and Forchheimer equation, using finite volume method. Then the mathematical simulator model of the distribution of non-cohesive sediment concentration in the body of dam was developed, based on the model output and sediment transport equation, using F.V.M. The model determines the value of the sediment concentrations at different points of the body on the basis of the gridding from the previous stage. The experimental results were used to evaluate the output of these models. The comparison of the experimental and numerical results seemed to verify the accuracy of the numerical model. The mean value of the relative error of sediments was calculated to be 8.14 percent as determined by the comparison made between the measured data on sediment distribution and the values calculated in three sections of the body of dam.

A large number of flows encountered in nature and technology are a mixture of phases. Advances in computational fluid mechanics have provided the basis for further insight into the dynamics of multiphase flows. Currently there are two approaches for the numerical calculation of multiphase flows: the Euler-Euler approach and the Euler-Lagrange approach
In the Euler-Euler approach, the different phases are treated mathematically as interpenetrating continua. In FLUENT, three different Euler-Euler multiphase models are available: the volume of fluid (VOF) model, the mixture model, and the Eulerian model. For sedimentation, we must use the Eulerian model. The Eulerian multiphase model in FLUENT allows for the modeling of multiple separate, yet interacting phases. The phases can be liquids, gases, or solids in nearly any combination. The Lagrangian discrete phase model (DPM) in FLUENT follows the Euler-Lagrange approach. The fluid phase is treated as a continuum by solving the time-averaged Navier-Stokes equations, while the dispersed phase is solved by tracking a large number of particles through the calculated flow field.
Sediment transport by fluid flow is one of the most important two phase flow in the nature. Due to existence of secondary current in channel bends, the mechanism of flow and sediment transport in these channels is much complex and locationg lateral intake at outer bank of the bens decreases this compelexity.
In this paper, mechanisms of sediments transport into the intake in a 180 degree bend channel with lateral intake have been simulated whit the Eulerian and Discrete phases models in fluent software. The intake is located at the outer bank of an 180o bend at position 115° with 45° diversion angle. The effect of diversion discharge rate and diversion angle on mechanism of sediment entry to the intake was considered.
The turbulence model is k-ε model. Model҆s in different time has performed and the result compared with laboratory result.The results show in Qr=40%, the mechanism of sediment entry was consist of continues entrance from downstream edge of intake and periodic entrance from upstream of the intake, however in Qr=25%, the mechanism of sediment entry was only consist of continues entrance from downstream edge of intake. The two models (Eulerian and Discrete phases) have shown good results. The rout mean square errors for outer boundary of the path of the particle at the channel ҆s bed for two discharges (25% and 40%) have measured.
The number of particle in discrete phases is limited; therefore this model cannot be display the depth of sediment. The Eulerian model displays the bed topography very well. Measuring mean square errors show that the model operation for topography simulation is very well. This model shows the location of intermittent dune and location of sediment accumulation very well. The discrete phase model can be shown the particle trapped place better than the Eulerian model.
Due to increase in intake discharge, dimension of sediment accumulation is decrease. the mechanism of sediment entry to lateral intake is affected by diversion angle of intake. the minimum sediment is entered to lateral intake at diversion angle equal to 50 degree.

In the present research, the developed 3D Eulerian-Lagrangian model of the sediment transport in Part A of this series is used to predict the effects of the bed-roughness height, water temperature (viscosity) and density of sediment grains. The selected parameters for the analyses are important both for sediment transport hydraulics and also uncertainties of experimental data. The results indicated that increasing bed-roughness height causes decrease in the saltation characteristics and consequently bed-load transport rate. The effects of water temperature (viscosity) are significant in the range of sand size, and it is necessary to keep it in the control for reducing uncertainties of experimental data. The sediment density has significant effects in both sand and gravel ranges, and thus the use of grains with different density compared to natural sediment can impose significant effects on experimental results.

In the present research, a 3D numerical model of the sediment grain movement has been developed based on the Eulerian-Lagrangian perspective. The forces which act on sediment grains are non-linear drag force, the shear lift force, the rotational lift (Magnus) force, the buoyancy force, the added mass force, the Basset history force and torque. Second-order nonlinear ordinary differential equations have been used to calculate linear and angular velocity vectors and also the position of sediment grains. Two parts including turbulent fluctuations in 3D space and a stochastic bed-particle collision model have been considered in the modeling procedure. The verification of the developed model has been examined for two important issues including the minimum number of jumps for statistical convergence and the appropriate value of the time step. The validation of the model has been performed using reliable experimental data in terms of the saltation height, length and streamwise particle velocity for the ranges of fine sand to coarse gravel. To evaluate the qualification of the conceptual model, the effects of the elimination of some forces and turbulent fluctuations, where do not exist consensus on the use of these factors in the previous researches, have been studied. The results indicated that the developed model can be considered as a numerical lab to study different aspects of the sediment transport in the prediction step, which will be performed in the part B of this series.

The mechanism of flow and sediment transport in channel bend is much complex. Because of secondary current, the sediment moves away from outer bank toward inner bank and therefore outer bank of the bend is one of best positions for lateral diversion. In this paper, the mechanism of sediment transport was simulated with SSIIM software in the U shape channel with lateral intake. In order to verify the numerical model results used in Montaseri’s lab studies, The position of injection was upstream of bend and sediment injection rate was approximately is equal to 250 gr/min and Frude number is equal to 0.32. The SSIIM numerical model solves the Navier-Stokes equations with the k-ε model on a three-dimensional. The bed load can be calculated Van Rijn’ formula. The numerical model has been implemented at various times to see how the formation and development of bed forms in the U shape channel with lateral intake. The numerical results show that the prediction of development of bed forms, mechanism of sediment entry to intake, location of intermittent dune and location of sediment accumulation are in fairly good agreement with experimental data and the maximum error occurred in front of intake.

Smoothed Particle Hydrodynamic method is a Lagrangian numerical particle method, best fitted for free surface hydrodynamics. SPHysics is one of the most well-known computer models based on SPH. However, this open source code is limited to one-phase Newtonian fluid problems. In this research, after developing the standard serial SPH code to a two-phase non-Newtonian model, the movable bed dam break problem has been modeled. An added pressure term is used in the pressure equation of state. Afterward, the model has been used for various problems and the validity of results was evaluated by related numerical models and experiments. Finally, the model has been challenged with modeling of dam break movable bed material by non-Newtonian Bingham model and compared with MPS method and experimental results.